For #11669, #11540, #11945, #11946, #11947.

Change-Id: Ifb0053c498cee9f3473c396f9338d82bd856c110
Reviewed-on: https://go-review.googlesource.com/12860Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>